Contents

Vista Variables in the Via Lactea (VVV) is a public IR variability survey of the Milky Way bulge and an adjacent section of the mid-plane where star formation activity is high. It will take 1929 hours, covering ~109 point sources within an area of 520 sq deg, including 33 known globular clusters and ~350 open clusters. The final products will be a deep IR atlas in 5 passbands and a catalogue of ~106 variable point sources. These will produce a 3-D map of the surveyed region (unlike single-epoch surveys that only give 2-D maps) using well-understood primary distance indicators such as RR Lyrae stars. It will yield important information on the ages of the populations.

The observations will be combined with data from MACHO, OGLE, EROS, VST, SPITZER, HST, CHANDRA, INTEGRAL, and ALMA for a complete understanding of the variable sources in the inner Milky Way. Several important implications for the history of the Milky Way, for globular cluster evolution, for the population census of the bulge and center, and for pulsation theory would follow from this survey.

Team

Our Team is composed by around 90 astronomers, and the Principal Investigators are Dante Minniti (DAA, PUC) and Philip Lucas (Centre for Astrophysics Research, University of Hertfordshire).

Observations

The strategy devised here allows to deliver interesting data to the community, enabling follow-ups throughout the survey. It also facilitates VISTA scheduling of other Public Surveys (e.g. VIKING) by alternating between the Bulge region (RA=17.0-18.75h) in year 3 and the Plane region (RA=11.7-17.4h) in year 4

cover the whole bulge and inner plane in ZY JHKs to 3-4 magnitudes deeper than 2MASS in the first season

find the first variable point sources in the second season

classify bulge and disk variable stars, microlenses, etc., and measure their amplitudes and periods in the third and fourth seasons

obtain final measurements based on light curves with 50-200 epochs for a large subset of the initial variables, and high proper motion objects at the end of the survey.

In order to cover the bulge between −100 < L < 100 and −100 < B < 50, and the disk between −100 < L < −650, and −20 < B < 20 we ask for a total of 192 nights over 5 years, distributed as shown in the Table below.

The final survey will cover 300 square degrees in the Galactic bulge and 220 square degrees in the inner plane. During the first year (2010) the whole bulge area will be mapped once at ZY JHKs, stepping through all 5 filters in each OB to provide near simultaneous fluxes and reliable colours in each field. It would also be observed once per night in Ks for 4 more epochs, and 2 more epochs in J. The strategy will be repeated for the whole plane field, alternatively through fields of varying density for optimal sky subtraction (see Tables 1 and 2 at the end).

Using only one filter per night maximizes returns, allowing to cover the whole areas, yielding deep ZY JHKs maps of the whole bulge and inner plane. Being fully aware of the confusion and background limits, the observing plan would circle alternatively through fields of varying density for optimal sky subtraction.

During the second year (2011) each field will be observed 3 epochs in Ks, to keep building the baseline. The observing strategy is designed to cover 300 sq deg per night. In that way the whole bulge and plane are observed again the second year, allowing the identification of variable sources (but not the phasing). These data will also allow the creation of deeper master maps in Ks, in order to fine tune the strategy for the main campaign of the following year.

The main bulge variability campaign is carried out during 75 nights in the third year (2012). We ask for consecutive nights during this season, although in practice there will be a few holes due to weather and to the unfortunate fact that 2-3 nights per month the Moon transits in front of the bulge. We will use the Ks-band to map the whole bulge and inner plane over and over (see Table 3 at the end). A subset of the fields can be observed more frequently (4 - 8 times per night). This strategy allows to partly remove aliasing and to improve the periods, while being more sensitive to smaller timescale variables and microlensing events. We have compared the specific advantages of doing the variability survey in J and Ks. On the one hand, the J-band gives better photometric precision, higher RR Lyr amplitudes (�J � 1.5�K), and deeper magnitudes. On the other hand, the Ks-band filter permits more coverage in heavily reddened regions, yields better distances, and gives tighter PSFs, on average by 0.1 arcsec, which would make a difference in the crowded regions. Based on photometry and light curve simulations we adopt the Ks filter for this proposal, noting that we would optimize the strategy after the first year’s ZY JHKs data are in hand.

During the fourth year (2013) the main plane variability campaign is carried out using 55 nights in Ks, following a similar strategy as in the case of the previous year.

Finally, during the fifth year (2014) we will observe for 20 and 14 nights the bulge and plane fields, respectively, but with observations spread over the season. This allows the measurement of longer-timescale variables, and the search for high-proper motion objects. A subset of the fields can be observed much more frequently (10-40 times per night). This strategy allows to find short-period variables and planetary transits.

We do not discard the possibility of extending the variability search for a sixth year if the coverage is incomplete due to scheduling or weather.